Line pitches smaller than approximately 66 nanometers (nm.) are beyond the theoretical capability of the 193 nm. immersion lithography optical systems used in conventional photolithography for single patterning. In order to achieve pitches smaller than 66 nm., technologies have been developed that exploit other features of the photolithographic process. One such technology is self aligned double patterning (SADP) which provides for an improvement in pitch by up to a factor of two.
In SAPD, a plurality of elongated, substantially parallel mandrels are formed on the upper surface of a first work surface. The mandrels are rectangular in cross-section with parallel sidewalls. Since there is no need to incur the added expenses of SAPD processing if the mandrels have a pitch more than twice the theoretical minimum pitch (hereinafter “TMP”) of the process used to form the mandrels, the mandrels ordinarily have a pitch that is greater than TMP by no more than a factor of two. For convenience, we will refer to pitches in the range between TMP and twice TMP as the SAPD range.
Spacers are formed on the work surface that extend from the sidewalls of each mandrel toward the two adjacent mandrels. The spacers are formed using essentially the same film and etch technology used to form spacers on the sidewalls of field effect transistor gates. Since the etching process is uniform, each spacer that is formed has approximately the same extension on the work surface from the sidewall of one mandrel toward the adjacent mandrel. The etching process is performed so as to leave a gap between the spacers near the midpoints between adjacent mandrels thereby exposing first portions of the first work surface.
The spacers are then used as masks in another etching process. First, the mandrels are removed to expose second portions of the work surface that underlie the mandrels. Then, a suitable etchant is used to etch the exposed portions of the first work surface, both the portions that were under the mandrels and the portions in the gaps near the midpoints between the mandrels, down to a second work surface. The remaining portions of the first work surface are then used as a mask to etch the second work surface down to a substrate, thereby forming trenches in the second work surface. The trenches are then filled with a metal such as copper. Since the trenches are formed both underneath the regions where the mandrels were located and underneath the gaps in the spacers near the midpoints between the mandrels, the metal routing has a pitch that is one-half the pitch of the mandrels and less than the TMP.
While the SAPD process enables the formation of a wire layout having a pitch that is one-half the pitch of the mandrels, the conventional SAPD process has the disadvantage that the spacing between adjacent wires is uniform since the extension of each of the spacers is the same. However, there are many situations in which it is desirable to be able to vary the wire spacing. For example, it frequently is desirable to increase the wire spacing so as to reduce same-metal cross-capacitance and thereby improve speed and AC power and/or reduce noise coupling between adjacent lines. It is also desirable to reduce same-metal cross-capacitance for edge-sensitive signals, such as clock signals. Thus, it is desirable to be able to vary the wire spacing to provide larger spacing for signals such as clock signals, high-fanout signals, speed critical signals, and asynchronous control signals.